1. Field of the Invention
The present invention relates generally to optical communications networks, and more particularly to an optical network where each network node includes an optical cross-connect system and an OAM (operations, administration and maintenance) signal is transmitted to control the cross-connect system of each optical node.
2. Description of the Related Art
Optical switching systems have been developed for purposes of eliminating the need for conversion from optical signals to electrical signals and vice versa at the end points of an optical transmission line. Substantial cost savings can be obtained by the direct optical cross-connect switching because the wide bandwidth of optical fibers would otherwise need a great number of opto-electrical and electro-optical converters. Maintenance of such high-capacity optical transmission systems is therefore an important factor for network providers to ensure reliable service. If a cable failure or a traffic overload occurs in an optical network, direct optical switching allows changeover to occur between very large groups of circuits in a short period of time and allows fast changeback when the situation has been restored to normal. Therefore, the quick recovery feature of the optical cross-connect system is a benefit for the maintenance of an optical network. Additionally, when protection switching occurs, optical cross-connect switching eliminates the need to provide time-division demultiplexing of high-capacity optical signals. The equipment needed for a network node can therefore be kept small.
On the other hand, the standard frame format known as SDH (Synchronous Digital Hierarchy) and SONET (Synchronous Optical NETwork) combines payload and OAM bits in a single frame. Since multiplexing is provided at the level of baseband signals and since a very large number of such frames are time-division multiplexed onto an optical signal, protection switching of optical channels would require time-division demultiplexing of optical signals down to baseband level in order to extract necessary OAM information. This would result in an increased recovery time and increased complexity. One solution would be to provide separate optical transmission lines for exclusively carrying OAM information. However, substantial cost would be needed to implement.